i'm limited to formline (and format), so no additional modules can be included.

Why are you limited & can't use modules?
If it's because this is homework, you should have told us is was.. If it's because you don't have root, be sure to Super Search for how to locally install modules (this comes up a lot).

Also, if you want the output string to be truncated to 10 chars, then use substr ... this demonstrates the difference:

while(<DATA>){
  chomp;
  print "== $_ ==\n";
  printf "printf: [%10s]\n", $_;
  printf "substr: [%10s]\n", substr($_, -10);
}
__DATA__
abc
abcdefghjiklmnopqrstuvwxyz
[download]